X-ray imaging is used in a wide variety of applications. Therein, an X-ray beam coming from an X-ray source is typically directed through a region of interest of an object and an X-ray detector is used to detect X-ray intensity of the X-ray beam after being transmitted through the object.
Generally, a generated X-ray image comprises a multiplicity of pixels arranged in a 2-dimensional matrix. For each of the pixels an X-ray intensity value is acquired using e.g. an X-ray detector. The X-ray detector may comprise one or a plurality of detector elements. For example, a number of detector elements may be identical to a number of pixels of the image and each detector element of the plurality of detector elements may acquire an X-ray intensity value for one of the pixels of the image. Alternatively, the detector comprises only a small number of detector elements compared to the number of pixels and may be scanned along a region of interest in order to acquire X-ray intensity values for each of the pixels of the image successively.
For example, the X-ray intensity values may be acquired simultaneously for each of these pixels, using an X-ray detector comprising a 2-dimensional matrix array of X-ray detector elements. Therein, for example each single X-ray detector element may provide the X-ray intensity value for one image pixel or a sum of signals from several X-ray detector elements may provide the X-ray intensity value for one single image pixel.
Alternatively, the X-ray intensity values for the multiple pixels may be acquired sequentially by scanning the X-ray beam and/or the X-ray detector through the region of interest. Therein, the detector may have one or a small number of detector elements which acquire X-ray intensity values for one single or a small number of pixels in one step and which is then scanned to a next position.
X-ray imaging may be particularly beneficial for medical applications. Therein, various interior structures in a body of a patient may be examined as such interior structures generally have different X-ray absorbing properties. For example, in mammography applications, structures within the tissue of a female breast may be examined in order to find any malicious tissue.
Energy-resolving X-ray imaging systems have been developed and are now becoming feasible for routine screening and clinical use. Such imaging systems are adapted for discriminating between photon energies of detected X-rays and hence access spectral X-ray information. In such energy-resolving X-ray imaging systems, an X-ray detector may not only measure an overall X-ray intensity impinging onto one of its detector elements but may furthermore be adapted to discriminate between the energies, i.e. the wavelength spectrum, of photons providing such impinging X-ray intensity.
For example, in energy-resolved X-ray imaging, one or several energy threshold value(s) or wavelength threshold value(s) may be predetermined before X-ray examination and during actual X-ray image acquisition a detector may then distinguish between portions of an entire impinging X-ray intensity having photon energies or wavelengths below such threshold value(s) and other portions of the overall X-ray intensity having photon energies/wavelengths above such threshold value(s). Such information may be valuable for subsequent image interpretation.
US 2010/232669 A1 discloses a method for dynamically optimizing the signal-to-noise ratio of attenuation data related to two different X-ray energies for reconstructing an image of an object under examination. The method comprises (a) estimating the thickness and the material composition of the object at a plurality of different projection angles, (b) for each of the various projection angles calculating for a variety of combinations of different first and second X-ray energies a corresponding common signal-to-noise ratio, (c) for each of the various projection angles choosing the first and the second X-ray energy causing the maximum corresponding common signal-to-noise ratio, and (d); for each of the various projection angles acquiring X-ray attenuation data of the object whereby the two X-ray energies are the X-ray energies causing a maximum signal-to-noise ratio assigned to the respective projection angle.
US 2010/301224 A1 discloses an X-ray imaging device including a polychromatic X-ray source and a detector having pixels suitable for operating in photon counting mode within at least one energy window bounded by at least one adjustable threshold, and at least one counter so that each pixel delivers an output dependent on the number of photons received by the pixel in the energy windows during a predetermined time interval. U.S. Pat. No. 8,442,184 A1 discloses a spectral CT with an energy-resolving detector array. WO 2013/093684 A2 discloses a photon-counting X-ray detector. US 2012/0087463 A1 discloses photon counting and energy discriminating detector threshold calibration.
The quality of an energy-resolved X-ray image and particularly of information about the spectral information comprised therein may depend on the specific setting of the threshold values. For example, improper setting of such threshold values may result in acquired energy-resolved X-ray images showing excessive noise. However, optimizing the setting of such threshold values has been found to be non-trivial as e.g. the object properties may vary substantially over the area or volume being imaged.